The production of light metals, e.g. magnesium and aluminum, by the electrolytic winning from their molten salts, is old and well documented. Similarly are the maintenance problems of such cells.
Refractories for use in magnesium reduction cell service must function in an extremely hostile environment. The agents potentially involved in chemical attack on the materials of construction include the molten salts, HCl, chlorine gas, steam and molten metal, all at temperatures above about 650.degree. C. Since magnesium metal is one of the more active metals in the periodic table toward oxidation it poses a significant problem in selecting materials of construction for production cells even when substantially free of the elements in a production cell, as for example molten metal handling and casting operations, and care must be taken in selecting material of construction because of the oxidation and reaction potential of these light metals. For example, refractories which survive in one area of the cell are unable to withstand direct contact with the molten or gaseous metal and vice versa.
In addition to the harsh chemical environment, large thermal gradients are maintained across refractories. Also, rapid thermal cycling occurs in operations such as feeding, metal removal and refractory repair. These thermal stresses accompanied by the physical intrusion of bath salts and vapors into the body of the refractory and the heater lines which, as the penetration deepens, undergoes numerous freeze-thaw cycles, act to fracture and break up refractory structures. It has been observed over the many years of evaluation and analytical investigation of refractory materials particularly in magnesium reduction cell service that these three factors, chemical attack, thermal shock and physical intrusion of salts, work in concert to degrade cell refractory materials. The various combinations of these elements in the different zones of the cell accelerate the physical deterioration of the materials of construction. Conventional cell materials are steel, graphite, alumina, beta alumina and alumina-silica. Steel and graphite are subject to oxidation and high heat conductivity. Therefore, where possible alumina and alumnia-silica constitute most of the refractory shapes, castables, mortars and the like used as components in cell construction.
During investigations of cell refractory failures, it was discovered that while the cell is producing magnesium metal at the electrode surface, it is producing aluminum metal and silicon metal in certain parts of the refractory system when such are composed of alumina and/or silica. Depending on its local environment, the silica undergoes reduction to the elemental state and may remain as Si.degree. or may alloy with magnesium to form magnesium silicide. All types of alumina investigated when exposed to molten magnesium convert, albiet not uniformly but locally, to aluminum metal and magnesium aluminum oxide spinel, MgAl.sub.2 O.sub.4 and if silicon is present the aforesaid reactions also occur. Futher, as in the case of silicon, some part of the aluminum will combine with elemental magnesium to form the usual alloys. It was initially thought that this reaction was driven by the electrochemical nature of the cell. It has now been discovered, however, that this is not the case. Experiments in which several different types of aluminas (.alpha., .beta. and .delta.) were exposed to molten magnesium metal or magnesium vapors outside a cell environment produced the same general result, for example, with regards to the alumina content of the refractory-MgAl.sub.2 O.sub.4 +Al.degree.. The extent of the reaction appears to depend upon the crystal form and surface area exposed. Thus, alumina with low surface areas and more stable crystal form, such as high density alpha aluminum oxide, are the most resistant to attack but even these will undergo some reaction in time.
A few researchers have reported similar reactions between aluminum metal and MgO refractories, Judd, M. S. and Nelson, J. A., "INTERACTION OF MOLTEN ALUMINUM AND MAGNESIUM OXIDE REFRACTORIES", Ceramic Bulletin, 55 No. 7 (1976) page 643; "ALUMINUM ALLOYS CONTAINING MAGNESIUM AND ALUMINUM OXIDE", Lindsay, J. G., Bakker, W. T. and Dewing, E. W., J. American Ceramic Society, 47 (1964) pages 90-94; and Jynge, H. and Motzfeldt, K. "REACTIONS BETWEEN MOLTEN MAGNESIUM AND REFRACTORY OXIDES", Electrochimica Acta, 25 (1980) page 139 reporting on the reaction of magnesium with aluminum oxide or amuninosilicates at temperatures higher than those normally used in a reduction cell. However, there seems to be no information in the literature concerning the exposure of refractories containing magnesium aluminum oxide spinel to molten metals nor their use in magnesium service or even a suggestion to that effect.